I'm reading that as you already have a laser designed for a rail mount?
In which case it is probably for a 20mm Weaver/M1913 Picatinny type rail.
Buy a length of "Weaver Rail", and find a way of fixing it under the comp tube. Ideally drill and tap and bolt it on. Otherwise, you might chop it to bolt on using a longer front trigger guard screw and some kind of spacer. Or use some very strong glue or maybe solder. Job done, to varying degrees of professionalism.
If you don't want the iron sights, you could stick it on top using the rather horrid Crosman Pro-Blox topped off with an "11mm to Weaver Adaptor Rail".
Or get a circular laser and a "Figure 8 Laser/Flashlight Clamp Mount" designed to put a laser/maglite on a scope. Ought to work.
Or buy a laser sight with a "Universal Trigger Guard Mount".
Bits in quotation marks are what you need to search for on sales/auction sites.